To meet operational requirements, it is often necessary to design the body of a vehicle that moves through a fluid with a configuration that generates an undesirable flowfield having detrimental consequences such as fluid flow separation and vortex formation, each of which contributes to the overall drag of the vehicle. One example of such a configuration is seen when providing the aft fuselage of an aircraft with an upswept shape to accept a cargo door and ramp.
The flowfield typically produced by the upswept fuselage is characterized by a three-dimensional boundary layer with significant cross-flow regions on the fuselage. This boundary layer separates into a pair of counter-rotating vortices, trailing downstream. The resultant flow is analogous to that associated with a missle at high angle of attack or a delta wing, but without the sharp leading edge.
The total drag resulting from this kind of flow can be split into two components. First, there is the pressure drag that arises because of the reduced pressure on the lower surface of the fuselage. In addition, there may be a considerable loss of flow energy in the form of rotational kinetic energy of the vortex structures and this is manifested as a vortex drag component. Depending on the geometry of the aircraft, the relative contribution of each drag component varies.
For reducing the drag associated with this kind of flowfield, the best approach is to attempt to optimize the geometry of the configuration at the design stage. Thus, high-up sweep angles should be avoided. Also, slender fuselages with little or no flatness in cross-section should be used since these minimize the area exposed to the low pressure. In many applications, however, it is not possible to optimize the geometry due to the need to meet structural and operational requirements, and significant amounts of vortex drag can result.
The use of strakes, as disclosed in U.S. Pat. No. 3,419,232 to McStay, et al., has been employed to reduce vortex drag. These strakes are essentially small vertical plates or fins placed beneath the fuselage and designed to be embedded in the vortex flow. They act to reduce the intensity of the swirl of the vortex structures and so reduce the vortex drag. However, the strakes contribute to drag associated with skin friction and carry a weight penalty resulting in increased fuel consumption as well as a reduced payload capacity.
Recently, increased emphasis has been directed at designing short takeoff and landing (STOL) aircraft having efficient cruise performance at relatively high speeds as well as having the ability to takeoff and land at slow speeds in a relatively short distance. To develop adequate lift for the aircraft during low speed operation, such aircraft commonly utilize powered-lift systems wherein the jet exhaust from the engines is diverted downward by a flap system on the lifting wing to increase lift.
However, advanced STOL aircraft employing powered-lift systems are able to fly at such low approach and takeoff speed that conventional aerodynamic control surfaces cannot adequately provide control forces for pitch, roll, and yaw inputs due to the associated low dynamic pressures. The conventional approach to this problem has been to provide the STOL aircraft with enlarged control surfaces positioned as far from the center of gravity as possible and having double-element rudders and elevators to obtain increased aerodynamic force. This solution results in added weight, complexity in operation, increased unreliability due to complexity, and increased drag at cruise speed due to the large size of the control surfaces. An alternative approach involves the use of thrust nozzles mounted at the ends of the fuselage and wings. Again, this solution results in added complexity and weight as well as reduction in available engine thrust, the source of propulsion for the thrust nozzles.
In general, solutions to the cruise drag and STOL control surface problems have been treated individually, not in a synergistic manner wherein one common device may resolve both problems.